You don't need to buy ANY of the April POTM cards
Ummm, you do know that you CANT complete the Optional Collection UNTIL you get past Rizzo?
There are TWO collections. First one is to Collect Five POTM cards, which you HAVE to do to get Rizzo, and they give you Five packs to get those cards.
Then after you get past Rizzo; you get the last 4 of the 9 total POTM players. Those 4 go towards the Optional which is 9 cards.
They made the last four non sellable so you had to pick the remaining four and couldn't pick doubles of ones you already locked in.